The present invention relates to polymeric materials and composites made by frontal polymerization More particularly, the present invention relates to functionally gradient polymers formed by frontal polymerization.
Functionally gradient or graded materials (FGMs) are materials whose composition varies spatially in a controlled manner. Many different methods have been devised for forming functionally gradient materials. In one such process, centrifugal force was used to prepare gradients in a carbon fiber reinforced epoxy composite to produce composites with spatially varying conductivity. Additionally, several researchers have done work on preparing gradient interpenetrating polymer networks (IPNs). Most of the work developed by these individuals involves producing a gradient by diffusing one component into another pregelled component followed by curing, or producing a gradient in the polymer using a gradient of illumination. The diffusing method can require as much as 280 hours to produce a gradient over 10.mu.. Using the absorption of light to produce a gradient is limited to polymers with a thickness less than 1 mm. None of these techniques can be used to produce gradients in polymers which are several centimeters in thickness.
Graded polymeric materials, such as Graded Refractive Index (GRIN) materials have found wide use in optical applications. These materials are prepared via interfacial gel polymerization, which is a slow process limited to producing gradients less than about 1 centimeter.
Another type of gradient material with definite utility is an optical limiter based on a gradient of nonlinear optical dye dissolved in a polymer matrix. An optical limiter is a device that strongly attenuates intense optical beams but allows high transmittance at low level light. Such a device would be very useful for protecting human eyes from intense laser pulses. A discussion of the types of organic materials that exhibits such nonlinear absorption is contained in Perry et al., "Organic Optical Limiter with a Strong Nonlinear Absorptive Response," Science, 1996, pages 1533-1536. They found that metallophthalocyanine (M-Pc) complexes containing heavy central atoms work well. These dyes are compatible with poly(methyl methacrylate) and dissolve in the monomer. This affords the great advantage of inexpensive materials.
Frontal polymerization is a method for converting monomer into polymer via a localized reaction zone that propagates through the coupling of the heat released by the polymerization reaction and thermal diffusion. Frontal polymerization was first discovered at the Institute of Chemical Physics in Chemogolovka, Russia by Chechilo and Enikolopyan in 1972. Polymerization fronts can exist with free-radical polymerization of mono- and multifunctional acrylates or epoxy curing. Frontal polymerization can be achieved in solution polymerization with monomers such as acrylamide, methacrylic acid and acrylic acid in solvents such as water and DMSO.
Frontal polymerization reactions are relatively easy to perform. In the simplest case, a test tube is filled with the reactants. The front is ignited by applying heat to one end of the tube with an electric heater. The position of the front is obvious because of the difference in the optical properties of polymer and monomer. Under most cases, a plot of the front position versus time produces a straight line whose slope is the front velocity. The velocity can be affected by the initiator type and concentration but is on the order of centimeters per minute.
The defining feature of thermal frontal polymerization is the sharp temperature gradient present in the front. The temperature can jump about 200.degree. C. over a little as a few millimeters, which corresponds to polymerization in a few seconds at that point.
In view of the foregoing, it would be a significant advancement in the art to provide a process for forming functionally gradient polymers which had a short reaction time and which could produce polymers several centimeters in thickness. Such a process and the polymeric materials created thereby are disclosed herein.